Slowly Adapting Mechanoreceptors Research Articles

Genetic Tracing of Cav3.2 T-Type Calcium Channel Expression in the Peripheral Nervous System.

Characterizing the distinct functions of the T-type ion channel subunits Cav3.1, 3.2 or 3.3 has proven difficult due to their highly conserved amino-acid sequences and the lack of pharmacological blockers specific for each subunit. To precisely determine the expression pattern of the Cav3.2 channel in the nervous system we generated two knock-in mouse strains that express EGFP or Cre recombinase under the control of the Cav3.2 gene promoter. We show that in the brains of these animals, the Cav3.2 channel is predominantly expressed in the dentate gyrus of the hippocampus. In the peripheral nervous system, the activation of the promoter starts at E9.5 in neural crest cells that will give rise to dorsal root ganglia (DRG) neurons, but not sympathetic neurons. As development progresses the number of DRG cells expressing the Cav3.2 channel reaches around 7% of the DRG at E16.5, and remains constant until E18.5. Characterization of sensory neuron subpopulations at E18.5 showed that EGFP+ cells are a heterogeneous population consisting mainly of TrkB+ and TrkC+ cells, while only a small percentage of DRG cells were TrkA+. Genetic tracing of the sensory nerve end-organ innervation of the skin showed that the activity of the Cav3.2 channel promoter in sensory progenitors marks many mechanoreceptor and nociceptor endings, but spares slowly adapting mechanoreceptors with endings associated with Merkel cells. Our genetic analysis reveals for the first time that progenitors that express the Cav3.2 T-type calcium channel, defines a sensory specific lineage that populates a large proportion of the DRG. Using our Cav3.2-Cre mice together with AAV viruses containing a conditional fluorescent reporter (tdTomato) we could also show that Cre expression is largely restricted to two functionally distinct sensory neuron types in the adult ganglia. Cav3.2 positive neurons innervating the skin were found to only form lanceolate endings on hair follicles and are probably identical to D-hair receptors. A second population of nociceptive sensory neurons expressing the Cav3.2 gene was found to be positive for the calcitonin-gene related peptide but these neurons are deep tissue nociceptors that do not innervate the skin.

Dual Innervation of Neonatal Merkel Cells in Mouse Touch Domes

Merkel cell-neurite complexes are specialized mechanosensory end organs that mediate discriminative touch sensation. It is well established that type I slowly adapting (SAI) mechanoreceptors, which express neural filament heavy chain (NFH), innervate Merkel cells. It was previously shown that neurotrophic factor NT3 and its receptor TrkC play crucial roles in controlling touch dome Merkel cell innervation of NFH+ fibers. In addition, nerve fibers expressing another neurotrophic tyrosine receptor kinase (NTRK), Ret, innervate touch dome Merkel cells as well. However, the relationship between afferents responsive to NT3/TrkC signaling and those expressing Ret is unclear. It is also controversial if these Ret+ fibers belong to the early or late Ret+ DRG neurons, which are defined based on the co-expression and developmental dependence of TrkA. To address these questions, we genetically traced Ret+ and TrkC+ fibers and analyzed their developmental dependence on TrkA. We found that Merkel cells in neonatal mouse touch domes receive innervation of two types of fibers: one group is Ret+, while the other subset expresses TrkC and NFH. In addition, Ret+ fibers depend on TrkA for their survival and normal innervation whereas NFH+ Merkel cell innervating fibers are almost unaltered in TrkA mutant mice, supporting that Ret+ and NFH+/TrkC+ afferents are two distinct groups. Ret signaling, on the other hand, plays a minor role for the innervation of neonatal touch domes. In contrast, Merkel cells in the glabrous skin are mainly contacted by NFH+/TrkC+ afferents. Taken together, our results suggest that neonatal Merkel cells around hair follicles receive dual innervation while Merkel cells in the glabrous skin are mainly innervated by only SAI mechanoreceptors. In addition, our results suggest that neonatal Ret+ Merkel cell innervating fibers most likely belong to the late but not early Ret+ DRG neurons.

Stomatin‐domain protein interactions with acid‐sensing ion channels modulate nociceptor mechanosensitivity

Acid-sensing ion channels (ASICs) and their interaction partners of the stomatin family have all been implicated in sensory transduction. Single gene deletion of asic3, asic2, stomatin, or stoml3 all result in deficits in the mechanosensitivity of distinct cutaneous afferents in the mouse. Here, we generated asic3−/−:stomatin−/−, asic3−/−:stoml3−/− and asic2−/−:stomatin−/− double mutant mice to characterize the functional consequences of stomatin–ASIC protein interactions on sensory afferent mechanosensitivity. The absence of ASIC3 led to a clear increase in mechanosensitivity in rapidly adapting mechanoreceptors (RAMs) and a decrease in the mechanosensitivity in both Aδ- and C-fibre nociceptors. The increased mechanosensitivity of RAMs could be accounted for by a loss of adaptation which could be mimicked by local application of APETx2 a toxin that specifically blocks ASIC3. There is a substantial loss of mechanosensitivity in stoml3−/− mice in which ∼35% of the myelinated fibres lack a mechanosensitive receptive field and this phenotype was found to be identical in asic3−/−:stoml3−/− mutant mice. However, Aδ-nociceptors showed much reduced mechanosensitivity in asic3−/−:stoml3−/− mutant mice compared to asic3−/− controls. Interestingly, in asic2−/−:stomatin−/− mutant mice many Aδ-nociceptors completely lost their mechanosensitivity which was not observed in asic2−/− or stomatin−/− mice. Examination of stomatin−/−:stoml3−/− mutant mice indicated that a stomatin/STOML3 interaction is unlikely to account for the greater Aδ-nociceptor deficits in double mutant mice. A key finding from these studies is that the loss of stomatin or STOML3 in asic3−/− or asic2−/− mutant mice markedly exacerbates deficits in the mechanosensitivity of nociceptors without affecting mechanoreceptor function.

Mice mechanoreceptors are modulated by Cav3.2 T‐type calcium channels

Mechanoreceptors are sensory receptors that respond to mechanical displacement and provide the senses of touch, pressure, vibration and proprioception. Mice have three types of low-threshold mechanoreceptors: slowly adapting mechanoreceptors (SAMs), rapidly adapting mechanoreceptors (RAMs) and D-hair receptors (Driscoll & Tavernarakis, 2000). D-hair receptors are the end-terminal of type Aδ fibre axons and present a high sensitivity and ultra-rapid temporal detection of slowly moving stimuli. The mRNA expression for the T-type voltage-gated calcium channel Cav3.2 is the only marker defined for mechanoreceptors and found almost exclusively in D-hair receptors. The Cav3.2 gene encodes one of the three low-voltage-activated (LVA) Ca2+ channels (Cav3.1, Cav3.2 and Cav3.3) and are all expressed by sensory neurons in the DRG (Iftinca et al. 2007). Previous published studies proposed that Cav3.2 channel has a primary role in nociception (Bourinet et al. 2005). However, another research group that used miberfradil, a drug that blocks all T-type channels, suggested that the mechanosensitivity of D-hair receptors is reduced by blockade of LVA channels (Shin et al. 2003). For this reason the specific physiological properties of the Cav3.2 channel has not been accurately described. In a recent issue of The Journal of Physiology, Wang & Lewin (2011) conducted a study with the purpose of demonstrating the role of the T-type Cav3.2 channels. In this study, Cav3.2−/− mutant mice were used to directly address the role of this channel in sensory transduction of cutaneous mechanoreceptors and nociceptors. Mice were produced by gene targeting in mouse ES cells; the targeting vector was designed to delete the IS5 region of the murine Cacna1H gene, which resulted in deletion of exon 6, corresponding to amino acid residues 216 to 267. This method was helpful in examining the mechanosensitivity of nociceptors and mechanoreceptors in normal Cav3.2 mice and in Cav3.2−/− mutant mice, proving that nociceptors sensitivity was not altered when the Cav3.2 gene was suppressed; on the other hand, D-hair receptor sensibility was drastically impaired in their ability to encode the properties of moving stimuli, losing their high sensitivity and ability to ensure rapid temporal detection to skin movement. In addition the functional properties of other mechanoreceptors and nociceptors were unaffected by the suppression of Cav3.2 channels. Why is this important? These new data provide conclusive results of the specific function of the T-type Cav3.2 channel, solving the previous controversy about its physiological function and proving that this specific gene is responsible for providing the functional and distinctive properties of D-hair receptors. Thus, this study encloses the specific activity of this gene, being now considered not only a high specific marker of D-hair receptors but also indispensable to maintain their high sensitivity in the detection of skin deflection. The expression of Cav3.2 in D-hair mechanoreceptors may serve to provide highly accurate temporal information about rapidly changing stimuli. This information helps humans to make fast and precise judgments about the speed and direction of tactile stimuli, a characteristic that can be considered vital for survival and functionality. This is the first time that the regulation for temporal coding in mouse mechanoreceptors from Cav3.2 T-type channels has been proved. Nevertheless, results from this study showed that there exists a possibility that a minor sub-population of nociceptors is dependent on the presence of Cav3.2 channels. The research revealed no certainty that such a population exists, and it is highly unlikely that this kind of population could be identified. Further studies will be needed to ascertain if this is indeed a possibility.

Molecular Identification of Rapidly Adapting Mechanoreceptors and Their Developmental Dependence on Ret Signaling

In mammals, the first step in the perception of form and texture is the activation of trigeminal or dorsal root ganglion (DRG) mechanosensory neurons, which are classified as either rapidly (RA) or slowly adapting (SA) according to their rates of adaptation to sustained stimuli. The molecular identities and mechanisms of development of RA and SA mechanoreceptors are largely unknown. We found that the "early Ret(+)" DRG neurons are RA mechanoreceptors, which form Meissner corpuscles, Pacinian corpuscles, and longitudinal lanceolate endings. The central projections of these RA mechanoreceptors innervate layers III through V of the spinal cord and terminate within discrete subdomains of the dorsal column nuclei. Moreover, mice lacking Ret signaling components are devoid of Pacinian corpuscles and exhibit a dramatic disruption of RA mechanoreceptor projections to both the spinal cord and medulla. Thus, the early Ret(+) neurons are RA mechanoreceptors and Ret signaling is required for the assembly of neural circuits underlying touch perception.

The sensory mechanotransduction ion channel ASIC2 (acid sensitive ion channel 2) is regulated by neurotrophin availability

Almost all sensory neurons of the dorsal root ganglia have a mechanosensitive receptive field in the periphery. We have shown that the sensitivity to mechanical stimuli of a subset of sensory neurons that are slowly adapting mechanoreceptors (SAM) is strongly dependent on the availability of brain-derived neurotrophic factor (BDNF). Here we have investigated whether the ASIC2 sodium channel, recently shown by us to be necessary for normal SAM sensitivity, might be regulated by BDNF and thus partially account for the down-regulation of SAM sensitivity seen in BDNF deficient mice. We show that the mRNA for ASIC2 channels is reduced in the DRG of BDNF deficient mice indicating that BDNF might maintain its expression in vivo. We also made short-term cultures of sensory neurons from adult BDNF deficient mice and used a specific antibody to detect the presence of ASIC2 channels in different classes of sensory neurons. We observed that the channel protein was dramatically down-regulated selectively in medium and large diameter neurons and this expression could be rescued in a dose and time dependent manner by addition of BDNF to the culture (10–100 ng/ml). Drugs that block new transcription or protein synthesis also prevented the rescue effects of BDNF. We observed that ASIC2 channels were down-regulated in sensory neurons taken from neurotrophin-4 and neurotrophin-3 deficient mice; these effects might be due to a selective loss of neurons that normally express large amounts of ASIC2 channels. In summary, our data identify the ASIC2 channel as a target of BDNF signaling in vivo and suggest that the functional down-regulation of sensory mechanotransduction in BDNF deficient mice is in part due to loss of ASIC2 expression.

ATP suppresses activity in slowly adapting but not rapidly adapting mechanoreceptors in toad skin.

To test the possible role of ATP in transducing or modulating touch sensation, an isolated skin-nerve preparation from the toad, Bufo marinus, perfused on the inner side, was used to examine the effects of ATP on slowly adapting (SA) and rapidly adapting (RA) mechanoreceptors, identified by ramp and hold indentation of the skin. ATP (1-50 mM) did not generate impulse activity in either SA or RA mechanoreceptors nor increase their responses to mechanical stimulation. Instead, ATP (> or =5 mM) reversibly and selectively suppressed impulse discharge from SA mechanoreceptors. Our results indicate that while ATP may modulate the responses of specific touch receptors, it is not involved in transducing touch into nerve impulses.

Neurotrophin 4 Is Required for the Survival of a Subclass of Hair Follicle Receptors

Neurotrophin-4 (NT4) is the most recently discovered neurotrophic factor in mammals and, functionally, the least well understood. Here, we used mice that lack NT4 to determine whether NT4 is required for the survival of functionally identified subclasses of cutaneous sensory neurons. By using three independent methods of histological and electrophysiological analysis, we show that NT4 is specifically required for the survival of down hair (D-hair) receptors that innervate a subpopulation of hair follicles. All other functionally distinct types of afferents neurons innervating hairy skin were not affected in their survival or in their function. Previous studies have shown that BDNF is required for the mechanical sensitivity of slowly adapting (SA) mechanoreceptors but not for the postnatal survival of myelinated cutaneous afferent fibers. In contrast, the receptive properties of SA mechanoreceptors were not impaired in animals lacking NT4. Consequently, these data show that the two trkB ligands, NT4 and BDNF, have distinct and nonoverlapping roles in supporting cutaneous sensory neurons. Whereas NT4 is required for the survival of D-hair receptors, BDNF supports the mechanical function of SA fibers.

A role for BDNF in mechanosensation.

Brain-derived neurotrophic factor (BDNF) is a survival factor for certain sensory neurons during development. Using electrophysiology in BDNF-deficient mice, we show here that slowly adapting mechanoreceptors (SAM), but not other types of cutaneous afferents, require BDNF in postnatal life for normal mechanotransduction. Neurons lacking BDNF did not die, but instead showed a profound and specific reduction in their mechanical sensitivity, which was quantitatively the same in BDNF -/- and BDNF +/- animals. Postnatal treatment of BDNF +/- mice with recombinant BDNF completely rescued the mechanosensitivity deficit. Therefore BDNF is important for regulating SAM mechanosensitivity, independent of any survival-promoting function.

Mechanical response properties of nociceptors innervating feline hairy skin.

1. The responses of feline cutaneous nociceptors were examined in vivo by systematically manipulating the intensive and spatial dimensions of mechanical stimulation. A computer-controlled motor was used to apply prescribed forces (5-90 g) to a nociceptor's receptive field, with flat-tipped, cylindrical probes of various sizes (contact areas: 0.1-5.0 mm2). The stimulating device and protocols were similar to those previously used to evaluate human perception, thus allowing for comparisons of the two data sets. 2. With a ramp-and-hold stimulus of controlled force, most nociceptors showed a slowly adapting (SA) response throughout the stimulus. In this way, nociceptors resembled low-threshold SA mechanoreceptors. However, in contrast to SA mechanoreceptors, nociceptors failed to exhibit an onset burst of activity associated with the stimulus ramp. Nineteen percent (6 of 31) of the nociceptors often showed the opposite trend during the stimulus, e.g., a gradually increasing firing rate. Most of these nociceptors (5 of 6) had particularly high mechanical thresholds. 3. With 30 stimuli repeated at short intervals (6-8 s), response rates tended to decrease across trials. This phenomenon was most evident with more intense stimuli. When two series of stimuli were separated by 4-5 min, there was no apparent trend of reduced responsiveness between series. 4. Overall, nociceptors responded in an orderly way to variations in force and probe size. For a given probe size, larger forces produced greater responses; for a given force, smaller probes produced greater responses. The relationship between probe size and force was best described as an even tradeoff between force and a linear dimension of the probe (i.e., probe perimeter), rather than the area of the probe. Thus a given pressure (force/area) did not evoke the same response from nociceptors as probe size was varied. 5. There were two significant differences in the mechanical responsiveness between A fiber and C fiber nociceptors. First, for a given set of stimuli, A fiber nociceptors exhibited a greater response rate than the C fiber nociceptors. Second, the A fiber nociceptors exhibited a greater differential response related to probe size than the C fiber nociceptors. On the basis of these two features, the A fiber nociceptors' response profiles showed a closer parallel with previously reported human pain thresholds than the C fiber nociceptors did. 6. When the nociceptors were subdivided as to their mechanical threshold, those with lower thresholds [mechanically sensitive afferents (MSAs)] showed a response saturation with the more intense stimuli. On average, the stimulus levels at which saturation occurred were close to human pain threshold. Those nociceptors with higher thresholds [mechanically insensitive afferents (MIAs)] did not show such saturation. Thus only the MIAs appeared to have the capacity to unambiguously encode mechanical stimulus intensities above pain threshold. The MSAs, on the other hand, exhibited their greatest dynamic response range near the threshold for nonpainful sharpness. Thus the group of afferents commonly defined as nociceptors exhibit a heterogeneity of mechanical response properties, which may serve functionally different roles for perception.

Rapidly and slowly adapting mechanoreceptors in the glans penis of the cat

Sixty-one single mechanoreceptive fibers were surgically isolated from the dorsal nerve of the penis in anesthetized, mature, and sexually intact male cats. Impulse activity was recorded extracellularly. Receptive fields on the glans penis were stimulated with an accurate computer controlled mechanostimulator. Thirty-four units were categorized as rapidly adapting (RA) based on the absence of a response to sustained skin displacement. The remaining were slowly adapting (SA) units responding to sustained displacement. Most of the SA units were located in the distal smooth glans whereas RA units predominated in the proximal spiny glans. Displacement thresholds were significantly lower for RA units. All units encoded indentation velocity although the SA units were better suited to discriminate slowly moving stimuli. In addition, SA units encoded sustained displacement amplitude. Male cats with a denervated glans penis display disoriented mounting behavior disabling intromission. The presence of distally located SA mechanoreceptors suggests they may be the primary penile proprioceptors, mediating, along with the RA mechanoreceptors, successful completion of intromission.

Location and discharge properties of respiratory vagal afferents in the newborn dog

Activity from afferents of respiratory origin was recorded from filaments of the vagus nerve in puppies 1–17 days old. Animals were anesthetized with a mixture of chloralose-urethane and artificially ventilated with the chest open. A total of 256 receptors were studied of which 96% were identified as slowly adapting mechanoreceptors (SARs) and 4% as rapidly adapting mechanoreceptors (RARs). Hence the newborn has fewer spontaneously active RARs than the adult. Thirty-five percent of SARs were located in the trachea and the remainder in the bronchi: a distribution similar to that found in the adult. The transpulmonary pressure (P tp) threshold required for SAR activation was higher in the newborn than the adult for both tracheal (2.42 cm H 2O vs 0.51 cm H 2O) and bronchial (4.66 cm H 2O vs 3.66 cm H 2O) receptors. This resulted in less SAR activity at FRC in the puppy (16% of the total number of SARs in 1–17-day-old animals) compared to the adult (63%). In addition, the discharge frequency of tracheal and bronchial SARs, at any given P tp, was lower in the newborn than in the adult dog. These data suggest that the newborn has considerably less volume-related feedback during the respiratory cycle compared to the adult.

Temperature-sensitive mechanoreceptors, thermoreceptors and heat nociceptors in the feathered skin of pigeons

1. Single unit recordings from afferent fibres of a cutaneous branch of the radial nerve were done during thermal stimulation of the skin of the wing with a thermode or with radiant heat or cold. The response of all cutaneous receptors to this thermal stimulation was studied. 2. Rapidly-adapting mechanoreceptors were never excited by thermal stimulation. 3. Slowly-adapting mechanoreceptors had characteristics similar to the type I slowly-adapting mechanoreceptor in mammals. There was an excitatory overshoot during rapid cooling and a transient inhibition during warming when activated by a thermode. The phasic response depended on the rate of temperature change and on the temperature range. Radiant heating or cooling was ineffective in eliciting a response in most cases. The static curves showed a broad maximum between about 36 and 43 °C. At temperatures above about 45 °C there was always a reduced activity. 4. Two receptors with thermoreceptor characteristics were found. One cold receptor showed increased activity at low skin temperature and one warm receptor was excited by moderate heating in the temperature range 35–40 °C. 5. Heat nociceptors were excited both by radiant heating and thermode stimulation. During staircase-like increases in skin temperature the mean threshold of the response of eight heat nociceptors was 47.1 ± 1.4 °C. There was a steep increase in firing rate up to at least 52 °C. Some of the heat nociceptors were also excited by noxious mechanical stimulation.

Physiological properties of neurons in the principal sensory trigeminal nucleus of the cat

The properties of primary afferent trigeminal fibers and neurons in the principal trigeminal nucleus were examined in decrebrate cats with the aim of identifying specific receptor categories represented among lemniscal neurons and possible transformations in second-order neurons. The main classes of sensitive mechanoreceptors innervating skin, hair, teeth and cornea were all found to be represented by modality-specific neurons except for some slowly-adapting mechanoreceptor neurons displaying thermal (cold) sensitivity. Neurons excited by joint receptors, nociceptors and thermoreceptors appear to be lacking. Multiply innervated specialized hairs (mystacial vibrissae and labial sinus sinus hairs) were represented by rapidly-adapting neurons with a range of velocity sensitivity and two classes of slowly-adapting mechanoreceptors homologous with types I and II; each neuron is excited by distinctive stimulus and response properties with little evidence of receptor interaction using natural stimuli. The properties of the “lemniscal” mechanoreceptive neurons of the principal V nucleus are compared with those of the pars caudalis of the spinal V nucleus; no obvious differences were evident in receptive field size, sensitivity or information transmission. These findings fail to provide support for the suggested distinction between “discriminative” and “crude” touch attributed to different sectors of the sensory V nuclear complex.

Properties of mechanosensitive neurons within Auerbach's plexus of the small intestine of the cat.

Three different kinds of mechanosensitive neurons were detected by direct electrical recording from Auerbach's plexus. Neurons classified as fast-adapting mechanoreceptors discharged spikes at the onset of stimulation, and the discharge stopped during a sustained stimulus of constant intensity. Slowly-adapting mechanoreceptors maintained during sustained stimulation, a steady discharge at a frequency that was a direct function of the intensity of the stimulus. Tonic type neurons responded to mechanical stimulation with a prolonged train of spikes which had a consistent pattern from preparation to preparation. Once the tonic-type cells were triggered, the discharge followed a characteristic time course that was unchanged by further increase or decrease in stimulus intensity. Tonic-type neurons are probably not first order sensory neurons, but they may be activated by input derived from primary mechanoreceptors. Frequency of discharge of slowly-adapting mechanoreceptors was increased by histamine. This appeared to be secondary to histamine-induced contractile activity of the musculature.

Neurophysiology of Ganglia of Auerbach's Plexus

The enteric plexuses of the automatic nervous system may be considered, on the basis of both function and morphology, to be a simple integrative nervous system of vertebrate animals. Microelectrcde studies of single unit activity within enteric ganglia reveal four distinct types of ganglion cells distinguished on the basis of pattern of spike discharge. These are (i) burst-type units which spontaneously discharge bursts of spikes at periodic intervals; (ii) fast- and slowly-adapting mechanoreceptors; (iii) tonic-type units which respond to mechanical stimulation with prolonged, all-or-nothing trains of spikes; (iv) single-spike units which spontaneously discharge single action potentials at variable intervals. The enteric plexuses are adapted for control of the intestinal musculature which behaves as an electrical syncytium activated by myogenic pacemaker potentials. The mechanism of neural control is integration of continuous neurogenic inhibition of the inherently excitable musculature.

An electropneumatic weight applicator for stimulating the slowly adapting mechanoreceptors in the cat's footpads.

A device is described for precise stimulation of the slowly adapting mechanoreceptors in the cat's footpads. At a preset delay following the arrival of an external triggering pulse, a selected weight in the range 50 gm-2 kgm is applied for a preset duration, then lifted pneumatically. A displacement transducer provides a voltage signal proportional to the displacement of the weight.

Response of trigeminal ganglion neurons to thermal stimulation of the beak in pigeons

The activity of neurons from the trigeminal ganglion of pigeons have been recorded while cooling or warming the beak. Thermosensitive neurons were seldom compared with mechanosensitive units. From a total of 16 thermosensitive neurons, 13 were excited by cooling and 3 by warming. The impulse frequency strongly depended on temperature. At constant temperatures, constant firing rates were established. The static curves of cold-sensitive units showed, that with decreasing temperature a nearly linear rise in firing rate occurred between about 36 °C and 20 °C. One quantified, warm-sensitive neuron showed, with increasing temperature, a nearly linear rise between about 35 °C and 44 °C. Sudden cooling or warming caused no pronounced overshoot as in mammals. Six cold-sensitive neurons were totally inhibited by rewarming as were 2 warm-sensitive units by cooling. No comparable influence of temperature on the discharge rate of some slowly-adapting mechanoreceptors (pressure stimulus) was exhibited.

Physiology of visceral afferent systems

Electrophysiological studies of single visceral afferent fibres are reviewed. Recordings were made from the vagus and pelvic nerves supplying the abdominal and pelvic viscera. Slowly-adapting mechanoreceptors, with non-myelinated afferent axons, end in the muscle coats of the hollow viscera. The receptors are excited by both passive distension and active contraction of the viscera. The strength and duration of visceral contractions, especially the bladder, can be accounted for by the “in series” arrangement of these mechanoreceptors. Rapidly-adapting visceral mechanoreceptors include the Pacinian corpuscle, with no known function, and the recently described “flow” receptor in the urethral mucosa.

NEURAL ACTIVITY IN MECHANORECEPTIVE CUTANEOUS AFFERENTS: STIMULUS-RESPONSE RELATIONS, WEBER FUNCTIONS, AND INFORMATION TRANSMISSION.

NEURAL ACTIVITY IN MECHANORECEPTIVE CUTANEOUS AFFERENTS: STIMULUS-RESPONSE RELATIONS, WEBER FUNCTIONS, AND INFORMATION TRANSMISSION.